How to merge 2 row cells in data table?

Question

This is the next step in my attempt to build a user-friendly transition matrix in R, a follow-on to post How to add a vertical line to the first column header in a data table?.

Running the MWE code at the bottom generates the transition table shown in the image below (with my comments overlaying). I'm trying to merge the top 2 cells (rows) in the left-most column and vertically-center the column header "to_state". Any suggestions for doing this? Using DT for table rendering if possible.

Please note that in the fuller code this MWE derives from, the table expands/contracts dynamically depending on the number of unique states detected in the underlying data.

I found good potential guidance in Shiny: Merge cells in DT::datatable, but it turns out in that case row cells in the body of the table (not header) are being merged so it is not applicable to my case.

I am not familiar with HTML, CSS. However, there are nice guidelines on-line for formatting HTML tables, including combined column/row mergers. See https://www.brainbell.com/tutorials/HTML_and_CSS/Combining_colspan_And_rowspan.htm, and https://www.w3schools.com/html/html_table_colspan_rowspan.asp. Makes me wonder if a better solution is to ditch my current DT/html combo and instead do the table completely in html where it seems there is more guidance for the rookie like me.

Here is the MWE code:

library(DT)
library(shiny)
library(dplyr)
library(htmltools)
library(data.table)

data <- 
  data.frame(
    ID = c(1,1,1,2,2,2,3,3,3),
    Period = c(1, 2, 3, 1, 2, 3, 1, 2, 3),
    Values = c(5, 10, 15, 0, 2, 4, 3, 6, 9),
    State = c("X0","X1","X2","X0","X2","X0", "X2","X1","X0")
  )

numTransit <- function(x, from=1, to=3){
  setDT(x)
  unique_state <- unique(x$State)
  all_states <- setDT(expand.grid(list(from_state = unique_state, to_state = unique_state)))
  dcast(x[, .(from_state = State[from], 
              to_state = State[to]), 
          by = ID]
        [,.N, c("from_state", "to_state")]
        [all_states,on = c("from_state", "to_state")], 
        to_state ~ from_state, value.var = "N"
  )
}

ui <- fluidPage(
  tags$head(tags$style(".datatables .display {margin-left: 0;}")), # < left-align the table
  h4(strong("Base data frame:")), 
  tableOutput("data"),
  h4(strong("Transition table inputs:")),
  numericInput("transFrom", "From period:", 1, min = 1, max = 3),
  numericInput("transTo", "To period:", 2, min = 1, max = 3),
  h4(strong("Output transition table:")), 
  DTOutput("resultsDT"),
)

server <- function(input, output, session) {
  results <- 
    reactive({
      results <- numTransit(data, input$transFrom, input$transTo) %>% 
        replace(is.na(.), 0) %>%
        bind_rows(summarise_all(., ~(if(is.numeric(.)) sum(.) else "Sum")))
      results <- cbind(results, Sum = rowSums(results[,-1]))
    })
  
  output$data <- renderTable(data)
  
  output$resultsDT <- renderDT(server=FALSE, {
    req(results())
    datatable(
      data = results(),
      rownames = FALSE,
      filter = 'none',
      container = tags$table(
        class = 'display',
        tags$thead(
          tags$tr(
            tags$th(colspan = 1, '', style = "border-right: solid 1px;"),
            tags$th(colspan = 10, sprintf('From state where initial period = %s', input$transFrom))
          ),
          tags$tr(
            mapply(tags$th, colnames(results()), style = sprintf("border-right: solid %spx;", c(1L, rep(0, ncol(results())-1L))), SIMPLIFY = FALSE)
          )
        )
      ),
      options = list(scrollX = F
                     , dom = 'ft'
                     , lengthChange = T
                     , pagingType = "numbers"  # hides Next and Previous buttons
                     , autoWidth = T
                     , info = FALSE #  hide the "Showing 1 of 2..." at bottom of table
                     , searching = FALSE  # removes search box
      ),
      class = "display"
    ) %>%
      formatStyle(c(1), `border-right` = "solid 1px")
  })
  
}

shinyApp(ui, server)

Answer 1

The first cell text should be in the upper header, not in the second.

library(DT)
library(shiny)
library(dplyr)
library(htmltools)
library(data.table)

data <- 
  data.frame(
    ID = c(1,1,1,2,2,2,3,3,3),
    Period = c(1, 2, 3, 1, 2, 3, 1, 2, 3),
    Values = c(5, 10, 15, 0, 2, 4, 3, 6, 9),
    State = c("X0","X1","X2","X0","X2","X0", "X2","X1","X0")
  )

numTransit <- function(x, from=1, to=3){
  setDT(x)
  unique_state <- unique(x$State)
  all_states <- setDT(expand.grid(list(from_state = unique_state, to_state = unique_state)))
  dcast(x[, .(from_state = State[from], 
              to_state = State[to]), 
          by = ID]
        [,.N, c("from_state", "to_state")]
        [all_states,on = c("from_state", "to_state")], 
        to_state ~ from_state, value.var = "N"
  )
}

ui <- fluidPage(
  tags$head(tags$style(".datatables .display {margin-left: 0;}")), # < left-align the table
  h4(strong("Base data frame:")), 
  tableOutput("data"),
  h4(strong("Transition table inputs:")),
  numericInput("transFrom", "From period:", 1, min = 1, max = 3),
  numericInput("transTo", "To period:", 2, min = 1, max = 3),
  h4(strong("Output transition table:")), 
  DTOutput("resultsDT"),
)

server <- function(input, output, session) {
  results <- 
    reactive({
      results <- numTransit(data, input$transFrom, input$transTo) %>% 
        replace(is.na(.), 0) %>%
        bind_rows(summarise_all(., ~(if(is.numeric(.)) sum(.) else "Sum")))
      results <- cbind(results, Sum = rowSums(results[,-1]))
    })
  
  output$data <- renderTable(data)
  
  output$resultsDT <- renderDT(server=FALSE, {
    req(results())
    datatable(
      data = results(),
      rownames = FALSE,
      filter = 'none',
      container = tags$table(
        class = 'display',
        tags$thead(
          tags$tr(
            tags$th(rowspan = 2, colnames(results())[1], style = "border-right: solid 1px;"),
            tags$th(colspan = 10, sprintf('From state where initial period = %s', input$transFrom))
          ),
          tags$tr(
            mapply(tags$th, colnames(results())[-1], style = sprintf("border-right: solid %spx;", rep(0, ncol(results()) - 1L)), SIMPLIFY = FALSE)
          )
        )
      ),
      options = list(scrollX = F
                     , dom = 'ft'
                     , lengthChange = T
                     , pagingType = "numbers"  # hides Next and Previous buttons
                     , autoWidth = T
                     , info = FALSE #  hide the "Showing 1 of 2..." at bottom of table
                     , searching = FALSE  # removes search box
      ),
      class = "display"
    ) %>%
      formatStyle(c(1), `border-right` = "solid 1px")
  })
  
}

shinyApp(ui, server)